The invention relates to a device for the induction heating of the inside of cylinder liners of reciprocating-piston engines for the purpose of hardening, as disclosed, for example, from G.B. patent specification No. 717,620.
The known hardening device has an inductor head insertable axially into the cylinder liner, with an axially aligned inductor loop and with a spray head directly adjacent in the circumferential direction. The inductor head is centered with play within the cylinder liner by means of bars arranged in a spoke-like manner. For hardening, the cylinder liner is rotated slowly past the fixedly located and immovably held inductor head the inductor loop and the quenching spray, so that a hardened strip, continuous in the circumferential direction, forms on the inner surface of the cylinder liner.
A disadvantage of the known hardening device is that, as a result of the necessary play between the centering bars for the inductor head and the inner surface of the cylinder liner, the position of the inductor loop relative to the inner surface is indeterminate to the extent of this play, and the inductive coupling of the cylinder wall to the inductor loop varies correspondingly. Such variation can be compensated only if, during the hardening operation, the working parameters are selected in such a way that it is unimportant. However, only very great hardening depth can be obtained in this way, which in turn leads to relatively pronounced hardening distortions that subsequently have to be ground out again in a way involving a high outlay which is uneconomical. In some circumstances, a great hardening depth can also result in surface melting and subsequent cracking. Consequently, to the applicant's knowledge, it has not hitherto been possible for the induction hardening of cylinder liners to gain acceptance in series production.
The object of the present invention is to develop the generic device such that hardening is accomplished with only low distortion, and in a manner that is practicable in engine operation; any residual distortions must be so slight that they can be ground out without appreciable extra outlay by conventional honing which can be carried out under series production conditions.
This and other objects and advantages achieved by the induction heating device according to the invention, in which the inductor head is fixedly clamped within the cylinder liner, and the heating is executed only at a relative standstill. This technique ensures every time, during heating, an exactly defined play-free relative position of the inductor loop and inner surface, so that the work can be carried out with finely adjusted working parameters. It is thereby possible to select the parameters so that the hardening depth is restricted to a thin boundary layer which, on the one hand, remains distortion-free (or at least of very low distortion) and on the other hand can do without extraneous quenching, since the non-heated basic material lying underneath it can itself quench the heated boundary layer through heat dissipation. Because extraneous quenching is unnecessary the inductor head remains very clean, and can correspondingly be positioned very exactly.
For the complete hardening of a cylinder liner, only individual hard strips are laid in place, which have a spacing in the circumferential direction and are separated from one another by non-hardened intermediate strips. These individual strips are applied successively in stages or steps. As a result of the thin boundary-layer hardening and because the hardness zone applied is broken down into strips, the distortions occurring during hardening are negligible, or at least so slight that they can be ground out again by honing, at a quite justifiable outlay. Also, the hardness zone being broken down into strips affords a load-bearing capacity sufficiently high in engine operation. The strips can remain restricted to the region which the piston rings of the reciprocating piston occupy in the region of the top dead-center.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.